If, like me, you find the default white zip file icon in 10.5 bland and boring, then it is quite easy to change it if you are using the built-in Archive Utility:

Open up Finder and go to /System/Library/CoreServices.

Find the Archive Utility, Control-click on it and select Show Package Contents from the pop-up menu.

Navigate into Contents » Resources.

Make a backup copy of bah-zip.icns (or any of the other compressed icons you'd like to change) by dragging the file to a folder in your user's home folder. The system will create a copy.

Find an icon you'd like to use, and copy it into this folder, renaming it to bah-zip.icns. I chose to copy the nice green bah.icns file from this same directory, renamed it to bah-zip.icns, and then copied it back in. (You will need to enter your admin password to modify this folder.)

Restart Finder or logout/login, and you will now have a nice customized zip icon for all your zip files.

[robg adds: As you're modifying a system-owned file, future system updates may overwrite your changes, of course. I haven't tested this one. If you're using 10.4, this hint should also work, though the name of the program to modify is BOMArchiveHelper, not Archive Utility. I haven't tested this one.]